The present invention relates to an apparatus for the sulfatizing roasting of a finely-divided selenium-containing raw material, especially the anode slime from copper electrolysis, with sulfuric acid, at a raised temperature; the apparatus includes a furnace chamber having a feed pipe for feeding a slurry composed of a finely-divided selenium-containing raw material and sulfuric acid into the furnace chamber, an outlet for withdrawing the roasted solid material from the furnace chamber, an outlet for removing from the furnace chamber the selenium-bearing gases produced during the sulfatizing roasting, members for heating the slurry in the furnace chamber, and devices for conveying, as a layer, the slurry fed into the furnace chamber from the feeding point of the slurry towards the outlet for solid material.
Several processes have been developed for industrial use for the production of selenium from selenium-containing raw materials, especially the anode slime from copper electrolysis. Among these, the processes based on the roasting of the raw material are those most common in practice. The most important roasting methods are soda roasting and sulfating roasting. Soda roasting does give a good yield of selenium, but the process has a disadvantage in the multi-stage recovery of selenium from the alkali selenites and selanates produced in the absorption of the gases. Problems of work hygiene constitute another disadvantage.
Sulfuric acid roasting, i.e. sulfatizing roasting, is also a process used in industrial production. It is based on the following reaction: EQU Se+2H.sub.2 SO.sub.4 .revreaction.SeO.sub.2 +2SO.sub.2 +2H.sub.2 O
The gaseous products of reaction produced in the roasting are fed into absorption devices, in which the selenium dioxide is first dissolved as a selenous acid, whereafter it is reduced to elemental selenium by means of chemical precipitation. The process can also be controlled so that the gases produced in the roasting are immediately cooled in the absorption device, in which case the above reaction proceeds from the right to the left, yielding elemental selenium as a product.
The latter process and an apparatus for carrying it out have been introduced in Finnish Pat. No. 46 054. This is a batch process, in which a mixture of slime and sulfuric acid is batched into a closed muffel in shallow vessels; the muffel if heated externally to the temperature required by the roasting reaction, 500.degree.-700.degree. C. The reaction gases produced, SeO.sub.2, SO.sub.2, and H.sub.2 O, are immediately fed into a venturi washer, where they react at 60.degree.-80.degree. C., thereby producing elemental selenium.
The above-mentioned selenium furnace is technically well-suited for production, but its use is limited by its relatively low capacity for roasting selenium. A typical furnace batch is 400-700 kg of slurry, corresponding to a production of 40-200 kg of selenium per batch. The batching and discharging operations of the roasting furnace require manual labor, and the manual handling of sulfuric-acid bearing slurry is complicated, if the height or diameter of the furnace is increased in order to increase the batch size. For this reason it is necessary to increase the thickness of the batch layer to 100-300 mm, which results in a slower removal of the gaseous products of reaction. The retention time in selenium removal is respectively long. approx. 24-48 hours/batch. A raised temperature produces a sintering effect, which clogs the diffusion conduits required for the removal of the gas and thereby lengthens the reaction time.
For the above reasons the selenium-removing device of the muffel furnace type is in practice limited to slurry quantities of 50-80 t/year, and respectively several furnace units in parallel are required for increasing the production capacity.
Finnish Pat. No. 28 803 discloses a selenium furnace in which slurry is fed onto an endless belt as a thin layer. The slurry is conveyed on this belt through the furnace, which is heated indirectly and into which air is fed simultaneously in order to oxidize the selenium. In this apparatus the slurry is heated by means of heat of radiation, and the roasting and vaporization of selenium are performed using air or an oxygen-bearing gas, adding compounds, such as metal oxides, which accelerate the roasting, and avoiding the increasing of the sulfur content of the slurry. Such a process has proven to be relatively slow, and it has not been applied successfully on an industrial scale.
The object of the present invention is therefore to provide an apparatus for the sulfatizing roasting of a finely-divided selenium-containing raw material, especially the anode slime from the electrolysis of copper, with sulfuric acid at a raised temperature, an apparatus which is continuous-working, has a more effective transfer of heat into the slurry than previously, and thereby a higher capacity than previously.